[#37] Add support for MapBox tiles and others

The common map function has been extended to allow configuring different
options for the base layer. There are default presets for MapQuest and
MapBox, but any XYZ layer can be added configuration options. The
options are passed to the templates via a helper function and to the js
modules via data-module attributes. Full docs included.
[#37] Fix broken link on map widgets docs
Update csw.rst
[#37] Update attributions
Merge branch 'patch-1' of git://github.com/tomkralidis/ckanext-spatial into tomkralidis-patch-1
Merge branch 'tomkralidis-patch-1'